A dmf dampens out low frequency vibration and reduces stress on the drive train created by high torque diesels so it makes them nice and quiet.

I was going to replace mine with a solid flywheel but then the noise disappeared and it didn't affect starting and that was 18 mnths or so ago...I don't do that many miles but every six months I have a look at the starter motor to check for metal swarf as this is an indication that the dmf is beginning to break up. The rattling will be a broken spring or two residing within the dmf, it sounds terrible at idle or low rpm so unless starting is being seriously affected I wouldn't be overly concerned if my experience is anything to go by.